1. Field of the Invention
The present invention relates to a battery voltage detector.
Priority is claimed on Japanese Patent Application No. 2011-075305, filed Mar. 30, 2011, the content of which is incorporated herein by reference.
2. Description of the Related Art
As known, a vehicle, such as an electric car or a hybrid car, is provided with a motor, and a high-voltage and high-capacity battery that supplies power to the motor. The battery includes multiple battery cells connected in series. The battery cell includes a lithium-ion battery, a nickel-metal hydride battery, or the like. In related art, a voltage balance control of monitoring and equalizing cell voltages of the battery cells is performed to maintain the performance of the battery.
It is mainstream to use a flying-capacitor-type voltage detection circuit in order to detect a cell voltage because dedicated insulating power or insulating elements are unnecessary for the flying-capacitor-type voltage detection circuit. The flying-capacitor-type voltage detection circuit has a demerit in that a cell voltage of a battery cell to be measured is detected lower than a true value if a leak current of a flying capacitor or a sampling switch increases due to time degradation. Consequently, a precise cell voltage cannot be obtained, thereby causing a decrease in the precision of cell balance control in some cases.
To overcome the above demerit of the flying-capacitor-type voltage detection circuit, Japanese Patent Laid-Open Publication No. 2002-291167 discloses technique of detecting a cell voltage of each battery cell to be measured in different timing using a flying-capacitor-type voltage detection circuit, and estimating a voltage immediately after the start of charging the flying capacitor (i.e., a cell voltage regarded as a true value), based on the voltage attenuation characteristics of the cell voltage obtained as a result of the detection.
The detection value of the cell voltage, which is obtained from the flying-capacitor-type voltage detection circuit, includes not only errors caused by the aforementioned increase in leak current due to time degradation of the flying capacitor and each switch, but also drift errors caused by variations with time of resistance values of an input resistance of the flying capacitor, an on-resistance and an input circuit resistance of each switch, and the like. The drift errors are factors that cause the cell voltage to be detected to be lower than the true value.
The present invention has been made in view of the above situations. An object of the present invention is to provide a battery voltage detector that reduces drift errors caused by variations with time of resistance values of various resistances included in the cell voltage detection circuit, thereby enhancing precision of detecting a cell voltage.